If such kind of developing device is constructed that the electrostatic latent image produced on the record sheet is passed through the developing liquid within a short time, there occurs the difficult problem that a sufficiently good development can not be effected, more particularly there occurs an edge effect and the record sheet can not smoothly be fed through the developing liquid.
Attempts have heretofore been made to eliminate such problems. These attempts are mainly divided into the following two measures:
(1) provision is made of a developing electrode and PA1 (2) an improved technique is used to supply the developing liquid.
In the first measure, the developing electrode is arranged such that the presence thereof does not prevent the travel of the record sheet, so that the developing electrode is not permitted to come near the record or specifically within 1 mm to 2 mm. As a result, the concentration of a picture image produced on that part of the record sheet which is distant from the edge of the electrostatic charge image by at least several mm becomes low. Such measure, therefore, is not adequate to develop a picture image having a good contrast. In addition, the strength of the electric field directed from the electrostatic latent image produced on the record sheet to the developing electrode is so insufficient and the traveling speed of the record sheet is so low that the developing electrode is not efficient enough to effect a high speed development.
As the second measure, the developing liquid may be agitated or permitted to flow over the record sheet or sprayed as a jet stream against it. In this case, however, the toner particles dispersed in the developing liquid are only moved within that region of the developing liquid which is distant by 20.mu. from the electrostatic latent image produced on the record sheet. This second measure, therefore, can not efficiently replenish the developing liquid in a region near the electrostatic latent image produced on the record sheet.
On the basis of the above mentioned two measures, provision has heretofore been made of a developing device comprising a number of pairs of developing and squeezing rollers between which is passed the record sheet and the upper rollers of which function as developing electrodes. In such developing device, if the number of the pairs of the developing and squeezing rollers is increased, the concentration of the picture image developed on the record sheet is raised, thereby effecting development with fidelity. However, if the number of the pairs of the rollers is increased, the number of rotary mechanisms for driving these pairs of rollers must to be increased and, hence, the developing device as a whole becomes expensive.